The proposed research will test relations between two highly heritable DSM diagnoses (Attention Deficit/Hyperactivity Disorder and Conduct Disorder) and two highly heritable empirically based syndromes (Attention Problems and Aggressive Behavior). Through family study methodology, will seek to determine whether it is more informative to consider these conditions as categorically distinct, quantitative variables or as a combination of both. In years 4 and 5 of this award, proposals for family and twin studies using the combined categorical-quantitative approach developed in the initial project of this SDAC will be submitted for funding.